1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device such as DRAM comprising a capacitor, particularly, to a method of forming a capacitor insulating film (electric charge accumulating film) included in such a semiconductor device.
2. Description of the Related Art
A capacitor is an important element included in a semiconductor integrated circuit. For example, a transistor and a capacitor are used in combination in a dynamic random access memory (DRAM), which is a kind of a semiconductor memory device, so as to perform writing and reading of data. A capacitor is also used widely as an element for accumulating electric charges in another semiconductor integrated circuit.
A capacitor included in a semiconductor integrated circuit comprises a lower electrode provided by a semiconductor substrate or a conductor formed on the substrate, a capacitor insulating film laminated on the lower electrode, and an upper electrode laminated on the insulating film. It was customary in the past to use a silicon oxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4) for forming the capacitor insulating film included in the capacitor used in an integrated circuit.
With a rapid progress nowadays in the integration density of the semiconductor device and in the storing capacity of the memory device, it is required to form a capacitor having a large capacitance, i.e., a large accumulating capacity of electric charges in a small planar region. A first means to meet this requirement is that the thickness of the capacitor insulating film is decreased so as to increase the capacitance per effective unit area. A second means is to employ a three dimensional structure so as to increase the effective surface area of the capacitor. The second means includes, for example, a trench capacitor technique and a stacked capacitor technique. In the trench capacitor technique, a trench is formed on the lower electrode, e.g., a silicon substrate, to form a capacitor along the surface of the trench, thereby increasing the effective area of the capacitor. When it comes to the stacked capacitor technique, a plurality of capacitors are formed in a stacked fashion on a transistor so as to ensure a large capacitor area without sacrificing the degree of integration.
However, serious problems remain unsolved in the conventional techniques exemplified above. Specifically, a leakage current is increased with decrease in the thickness of the capacitor insulation film, making it impossible to decrease the thickness of the film to a level exceeding a certain level. It is also technically difficult to achieve a three dimensional structure of a further complex structure. It follows that it is difficult to provide a DRAM having a higher degree of integration as far as silicon oxide or silicon nitride is used for forming the capacitor insulating film. As a matter of fact, a DRAM having a degree of integration exceeding a level of giga bits has not yet been developed.
Under the circumstances, in order to achieve a further improved fineness and to further improve the integration density, it is absolutely necessary to use a dielectric material having a dielectric constant higher than that of the conventional insulating film for forming the capacitor insulating film. In recent years, it is studied to use high dielectric constant materials having a perovskite crystal structure such as strontium titanate (SiTiO.sub.3), barium strontium titanate (Ba.sub.x Sr.sub.1-x TiO.sub.3), and PZT (PbZr.sub.x Ti.sub.1-x O.sub.3), said highly dielectric materials having a dielectric constant higher than that of SiO.sub.2 or Si.sub.3 N.sub.4. The dielectric constant of these high dielectric constant materials is 20 to 1,000 times as high as that of silicon oxide (SiO.sub.2).
However, it is necessary to solve the problems given below in using these high dielectric constant materials of perovskite crystal structure for forming a capacitor insulating film.
In general, a highly dielectric film has a narrow forbidden band, with the result that a leakage current tends to flow when a voltage is applied to the film. It follows that, if the thickness of the high dielectric constant film is decreased in using the film for the capacitor insulating film of a DRAM in order to ensure a required capacitance, the leak current tends to be excessively increased. It should also be noted that the dielectric constant of a high dielectric constant film having a perovskite crystal structure tends to be lowered, if the thickness of the film is decreased. It follows that, even if the film is made thinner, the capacitance thereof is not sufficiently increased. Such being the situation, it is impossible to obtain a sufficiently large capacitance by simply using the high dielectric constant materials exemplified above for forming the capacitor insulating film, making it necessary to employ a three dimensional structure as in the trench capacitor technique and the stacked capacitor technique.
In employing a three dimensional structure, it is necessary to form a high dielectric constant thin film with a good step coverage on a surface having a recess or projection. However, the sputtering technique used in the conventional technique of forming a high dielectric constant thin film is incapable of forming such a thin film with a good step coverage. This makes it necessary to employ a chemical vapor deposition (CVD) method, which permits forming a thin film with a good step coverage, for forming the high dielectric constant thin film in place of the sputtering method. However, it is impossible to form uniformly by the known CVD method a thin film of the high dielectric constant material, which is a complex oxide compound, on a substrate having stepped portions with a good step coverage. Thus, it is difficult to form a capacitor of three dimensional structure by using a high dielectric constant thin film as a capacitor insulating film. As a result, the degree of integration achieved in the semiconductor device comprising a capacitor insulating film made of a highly dielectric material is not so high as that achieved in the semiconductor device comprising a capacitor insulating film made of SiO.sub.2 or Si.sub.3 N.sub.4.
To be more specific, an MOCVD (metal organic CVD) method using an organometalic compound as a raw material is employed, in general, for forming a metal oxide film by CVD method. The high dielectric constant material having a perovskite crystal structure, which is certainly a metal oxide, consists of several kinds of metal oxides. As a result, serious problems are generated as described below in the case of employing an MOCVD for forming a thin film of the high dielectric constant material. Specifically, in order to form a thin film having a high dielectric constant as desired, it is absolutely necessary to form the film such that the crystal structure of perovskite type is not disturbed. To meet this requirement, it is necessary to control the deviation of the crystal composition from the stoichiometric ratio to fall within a range of .+-.10%. Where it is necessary to control accurately the composition of the complex oxide film, the MOCVD is performed under mass transport limited conditions in which the thin film deposition rate is determined by the feed supply rate. Under the mass transport limited conditions, the thermal decomposition of the feed material is performed at a high rate, with the result that the thin film deposition rate is rendered proportional to the feed supply rate of the raw materials. It follows that the composition of the deposited complex oxide can be controlled accurately by accurately controlling the feed supply rate of each raw material during CVD performed under the mass transport limited conditions. The feed supply rate of each raw material can be controlled by accurately controlling the CVD conditions such as the raw material temperature, pressure in the raw material container, and flow rate of the raw material bubbling gas. The particular method is employed for forming dielectric thin films such as a film of Ba.sub.x Sr.sub.1-x TiO.sub.3 and high temperature superconductor films such as a film of YBa.sub.2 Cu.sub.3 O.sub.7-d.
The film composition can be controlled accurately by the CVD method performed under the mass transport limited conditions, as described above. However, the particular CVD method is not satisfactory in the step coverage of the deposited film. Specifically, under the mass transport limited conditions, the raw material is not expanded sufficiently on the surface of a substrate but is subjected to decomposition reaction immediately after reaching the substrate surface. It follows that, where the substrate has, for example, a trench structure on the surface, it is impossible to obtain a film of a uniform thickness because the raw material easily reaches some portions to deposit, but is unlikely to reach other portions of the trench structure. Such being the situation, the MOCVD under the mass transport limited conditions fails to achieve the object of employing a three dimensional structure such as a trench capacitor and a stacked capacitor, resulting in failure to comply with the demands in the giga bits generation.